1. Field of the Invention
This invention relates to electroconductive aramid paper and tape made therefrom suitable for electrostatic discharge interference and/or electromagnetic interference shielding.
2. Description of Related Art
NOMEX® Type 843 Conductive Carbon Blend aramid paper consists of NOMEX® brand floc and fibrids blended with conductive carbon fibers. This paper has been available in both hot calendered and uncalendered versions. The uncalendered version of this paper has a basis weight of about 40 g/m2, a density of about 0.29 g/cm3, and a tensile strength of about 16 N/cm, which corresponds to tensile index of 40 N*m/g, and can be easily saturated with polymer resins. However, it has been found that this paper does not have adequate tensile strength for automated tape winding of conductors, resulting in breakage and tearing of aramid tapes when wrapped using the under the tensions normally used by automatic winding devices. The hot calendered version of this paper has an improved tensile strength of about 35 N/cm (a tensile index about 90 N*m/g) and is strong enough for the automated tape winding; however, this calendered paper is less saturable and less formable, because after calendering the resulting paper is denser (about 0.64 g/cm3). The saturability of the paper is important for paper used as electrical insulation because in many applications the insulation is wrapped around a part, and the wrapped part is then impregnated with a polymer resin to substantially eliminate any air voids in the wrapping and to reduce the non-uniformity of electrical field and subsequent premature failure of the insulation. After the paper is wrapped around a part or another wrapping, the paper must be porous enough to allow polymeric resins to pass through the paper to fully impregnate both the paper and any other wrappings that might be present.
It is also desired that the conductive paper have a certain level of surface resistivity to avoid buildup of charge and provide an optimum electrical shielding in the particular application. Thus, a preferable surface resistivity of conductive tapes for the outside layers of the main wall insulation of coils in stators of high voltage motors is in the about 100 to 400 ohms/in2 range. Also, it is very important to have a manufacturing process which allows a good control of surface resistivity of the final paper. The surface resistivity of the hot calendered lightweight NOMEX® paper type 843 (about 700 ohms/in2 in the machine direction and about 1800 ohms/in2 in the cross direction) is about seven times that of the uncalendered paper (95 ohms/in2 in the machine direction and 250 ohms/in2 in the cross direction).
U.S. Pat. No. 2,999,788 to Morgan; U.S. Pat. No. 3,756,908 to Gross; and U.S. Pat. No. 4,481,060 to Hayes disclose papers based on fibrids from synthetic polymers including papers from aromatic polyamide (aramid) fibrids and their combination with different fibers.
U.S. Pat. No. 5,233,094 to Kirayoglu et al. discloses a process for making strong paper comprising 45-97% by weight of p-aramid fiber, 3-30% by weight of m-aramid fibrids and 0-30% by weight of quartz fiber. The paper is produced by forming, calendering, and additional high temperature heat treatment at least at 510° F. (266° C.).
U.S. Pat. No. 5,126,012 to Hendren et al. discloses high strength aramid paper from floc and fibrids, and carbon fiber is among the possible types of the floc. Necessary mechanical properties are achieved after hot compression of the paper in the press at a temperature of 279° C.
U.S. Pat. No. 5,316,839 to Kato et al. discloses multilayered aramid paper with conductive fibers in the conductive layer of the structure. The paper is prepared by forming followed by hot compression or hot calendering at or above the glass transition temperature of polymetaphenylene isophthalamide (275° C.).
Previously, aramid papers with conductive fillers required hot calendering or hot compression to make the paper stronger and thereby suitable for automated tape winding. At the same time, calendering or hot compression significantly changes the electrical properties of the paper, as well as reducing its free volume and ability to be saturated and impregnated by a resin. What is needed therefore is a conductive aramid paper suitable for making a tape that has the desired electrical properties, is saturable by resins, and is also strong enough to be processed in automated tape winding machines.